You need a 70% alcohol solution...

Question

You need a 70% alcohol solution. On hand, you have a 270 mL of a 15% alcohol mixture. How much pure alcohol will you need to add to obtain the desired solution?

You will need

 _____mL of pure alcohol

to obtain

______ mL of the desired 70% solution.

Answer 1

.15(270) + 1.00(x) = .7(270+x)

x - .7x = .7(270) - .15(270)

.3x = .55(270) = 148.5

x = 148.5/.3 = 495 ml of pure alcohol

mixed with 270 ml of 15% alcohol to yield

270+x = 270+495 = 765 ml of 70% alcohol

.15(270) = 495 = 40.5 +495 = 535.5 = .7(765)

100 is within 30 of 70

15 is within 55 of 70

495/270 = 11/6 = 55/30